A '''reverse telephone directory''' (also known as a '''gray pages''' directory, criss-cross directory or '''reverse phone lookup''') is a collection of telephone numbers and associated customer details. However, unlike a standard telephone directory
, where the user uses customer's details (such as name and address) in order to retrieve the telephone number of that person or business, a reverse telephone directory allows users to search by a telephone service number in order to retrieve the customer details for that service.
Reverse telephone directories are used by law enforcement and other emergency services in order to determine the origin of any request for assistance, however these systems include both publicly accessible (listed) and private (unlisted) services. As such, these directories are restricted to internal use only. Some forms of city directory|city directories
provide this form of lookup for listed services by phone number, along with address...